For many years the inductive-output linear-beam density-modulated electron tube has been a basic but neglected design since its development by A. V. Haeff in 1939. See "An Ultra High Frequency Power Amplifier of Novel Design" by A. V. Haeff, Electronics, February 1939; and "A Wideband Inductive Output Amplifier" by A. V. Haeff and L. S. Nergaard, Proceedings of the IRE, March, 1940. Haeff himself noted in his second paper the high interest then being generated by the contemporaneous work of the Varian brothers on velocity-modulated linear-beam microwave tubes. Such tubes, exemplified originally by the klystron, soon overwhelmed the field, since unlike the Haeff tube, they were not limited in frequency by electron transit time problems, nor was power limited by a grid. Consequently, no commercial applications of the Haeff tube have occurred during the past thirty or more years.
Nevertheless, the Haeff-type tube does have some advantages. In certain useful frequencies, especially the 100-300 megahertz band, it can be of much smaller length than a comparable klystron. In certain applications, especially as a linear amplifier in AM service, it can have a higher average efficiency. As in classical triodes, the electron beam current varies with the drive level. By contrast, in a conventional klystron the beam is invariant with drive level, so that it is comparatively less efficient at low signal levels.
Compared to a classical triode, the Haeff-type tube shares many of the advantages of klystrons, i.e., more power gain, simpler construction, output cavity at ground potential, and a collector which is separate from the output cavity and which can be made quite large for handling high waste beam power.
Such advantages, however, have been essentially unavailable due to the shortcomings of the Haeff-type tube, especially the comparative low output power heretofore possible. The earliest designs of Haeff produced about 10 watts CW output at 450 Mhz; later this was increased to 100 watts; beam voltages were at the 2 kilovolt level. However, these power levels are far short of practical requirements for modern communications and other applications. The Haeff-type tube has heretofore not been adaptable to higher power applications, and thus its advantages have continued to remain unavailable, particularly in applications, for example, television broadcasting, requiring kilowatt-level CW RF power and beyond. Generally the need has continued unfulfilled for a vacuum tube of compact design having the high efficiency and broadband characteristics for operations, especially in the 100-1000 MHz range and above, and especially at power levels in the kilowatt to megawatt CW range.